1. Field of the Invention
The present invention generally relates to tire retreading and repairing and more particularly to a method and apparatus for heating a pressure vessel to a temperature required for vulcanizing rubber so that a tire casing with a new tread or a tire casing with repairs placed in the pressure vessel in a manner well known in the art will be subjected to the required pressure and temperature without the heating medium being discharged into the vessel thereby enabling the heating medium to be circulated in a closed loop system thereby providing a more efficient tire retreading or repairing procedure.
2. Description of the Prior Art
Tire retreading and repairing procedures are well known in which the new tread or repair is subjected to pressure and temperature conditions within a vessel. In one presently used method, a strip of rubber is applied to a prepared casing for rebuilding or repairing a worn tire. The strip of rubber may be a pre-vulcanized tread, commonly referred to as a pre-cured tread, or the strip of rubber may be a strip of unvulcanized rubber. The pre-cured tread method is used extensively in the retreading of tires, particularly truck tires. In this method, a pre-cured strip of rubber is applied to a prepared casing with a thin strip of unvulcanized bonding layer, referred to as a cushion gum, being sandwiched between the casing and the tread. This cushion gum contains bonding chemicals and bonds or welds the new tread to the casing. In order to obtain a bond, the cushion gum must be subjected to a predetermined temperature and pressure with different systems requiring different temperatures and pressures. After the tread and cushion gum is applied to the prepared casing, an envelope in the form of an airtight flexible sheath is placed over the outside of the tread and tire which is then mounted on a wheel or rim and the envelope is sealed by the rim flange or in other various ways and the tire is inflated and placed in a pressure vessel.
In the system using unvulcanized strips of rubber, a strip of uncured or unvulcanized rubber is applied to a prepared casing which can be accomplished by extruding ribbons of rubber directly to the prepared casing or by applying a preextruded strip of rubber, either by hand or by machine. Cushion gum may or may not be used in this system. In any event, after the rubber is applied to the prepared casing, a rubber or flexible mold is put over the uncured rubber and it is then covered with an envelope in the same manner as in precured tread system, mounted on a wheel or rim with the envelope sealed, placed in the pressure vessel where it is subjected to pressure and heat for a predetermined time at which time the strip of rubber is bonded and molded.
The most popular method of applying heat in the pressure vessel is by the use of electrical heating elements. Another procedure employs the introduction of live steam into the pressure vessel. The use of electrical heating elements is very expensive and is dangerous because of potential fires. Electrical heating elements attain a temperature in excess of 900.degree. F. which requires that air must be constantly circulated over the heating elements. In most cases, the heating elements are located in an air duct in the pressure vessel which is above the tires. There have been occasions when a heating element falls and drops down into the duct and burns its way through the duct and onto the tires and the tires are ignited thus causing a dangerous fire situation. Electrical energy is quite expensive to use and pressure vessels which are heated with electrical energy cannot be preheated economically thus requiring considerable time in which to heat the pressure vessel after the prepared tires have been placed therein so that each "load" of tires placed in the pressure vessel requires considerable time for the tires to be subjected to the entire cycle of operation.
The following U.S. Patents relate to this subject matter but do not disclose a similar method or apparatus:
U.S. Pat. No. 2,025,993, Dec. 31, 1935 PA1 U.S. Pat. No. 3,329,748, July 4, 1967 PA1 U.S. Pat. No. 3,698,975, Oct. 17, 1972 PA1 U.S. Pat. No. 3,769,121, Oct. 30, 1973 PA1 U.S. Pat. No. 3,816,217, June 11, 1974 PA1 U.S. Pat. No. 4,370,283, Jan. 25, 1983